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A shifting assembly for shifting the reverse gear of a variable-speed
gearwheel transmission. The shifting assembly comprises a reverse-gear
gearwheel mounted to rotate on a transmission shaft which has a first
tooth array. A clutch body, that is rotationally fixed to the
transmission shaft which has a second tooth array, is associated with the
reverse-gear gearwheel. The first and the second tooth arrays are axially
forced into engagement in order to engage the reverse gear. A
synchronizer ring, connected to the reverse-gear gearwheel, has a
friction surface which engages another mating friction surface during
synchronization. The synchronizer ring and the reverse-gear gearwheel can
rotate relative to one another by a limited angular amount between a
blocking position and a through-connection position. A stressing element,
arranged between the synchronizer ring and the reverse-gear gearwheel,
pre-stresses the synchronizer ring relative to the reverse-gear gearwheel
in the direction of a blocked position.

Inventors:

Renner; Stefan; (Bodman-Ludwigshafen, DE)

Assignee:

ZF FRIEDRICHSHAFEN AGFriedrichshafenDE

Serial No.:

124893

Series Code:

13

Filed:

November 16, 2009

PCT Filed:

November 16, 2009

PCT NO:

PCT/EP2009/065203

371 Date:

April 19, 2011

Current U.S. Class:

74/339

Class at Publication:

74/339

International Class:

F16H 3/38 20060101 F16H003/38

Foreign Application Data

Date

Code

Application Number

Dec 15, 2008

DE

10 2008 054 665.8

Claims

1-13. (canceled)

14. A shifting assembly (1) for shifting a reverse gear of a
variable-speed gearwheel transmission, the shifting assembly comprising:
a reverse-gear gearwheel (4) arranged to rotate on a transmission shaft
(2) and having a first clutch tooth array (5); a clutch body (6)
connected in a rotationally fixed manner to the transmission shaft (2)
and having a second clutch tooth array (7) associated with the
reverse-gear gearwheel (4); the first and the second clutch tooth arrays
(5, 7) being brought into engagement with one another when engaging a
reverse gear; a synchronizer ring (8) having a friction surface being
associated with the reverse-gear gearwheel (4), and the synchronizer ring
(8) co-operating with a counterpart friction surface (16) during a
synchronization process; the synchronizer ring (8) and the reverse-gear
gearwheel (4) being rotatable relative to one another, by a limited
angular amount in a circumferential direction, between a blocked position
and a through-connection position; at least one stressing element (14)
being located between the synchronizer ring (8) and the reverse-gear
gearwheel (4) and being actively connected with the synchronizer ring (8)
and the reverse-gear gearwheel (4) in such manner that the synchronizer
ring (8) is pre-stressed relative to the reverse-gear gearwheel (4) in a
direction of the blocked position; and in the blocked position, at least
one blocking element (10) prevents the first and the second clutch tooth
arrays (5, 7) from engaging with one another by positive interlock means.

15. The shifting assembly according to claim 14, wherein the counterpart
friction surface (16) is located on a forward-gear gearwheel (15)
arranged adjacent to the reverse-gear gearwheel (4).

16. The shifting assembly according to claim 14, wherein the counterpart
friction surface (16) is formed on a synchronizer element (23) connected
in a rotationally fixed manner to the transmission shaft (2).

17. The shifting assembly according to claim 16, wherein the synchronizer
element (23) is a ring with a conical counterpart friction surface (16).

18. The shifting assembly according to claim 16, wherein the synchronizer
element (23) is formed integrally with the clutch body (6).

19. The shifting assembly according to claim 14, wherein the at least one
blocking element (10) comprises a plurality of blocking bolts (10)
distributed around a circumference of the synchronizer ring (8).

20. The shifting assembly according to claim 19, wherein at least one of
the cylindrical blocking bolts (10) has a cylindrical drive-pin (11), at
a free end thereof, with a smaller diameter than a remainder of the
blocking bolt (10) which projects into a bore (13) in the reverse-gear
gearwheel (4) associated with the drive-pin (11).

21. The shifting assembly according to claim 20, wherein a conical
blocking surface (12) forms a transition from a larger diameter of the
blocking bolt (10) to the smaller diameter of the drive-pin (11).

22. The shifting assembly according to claim 19, wherein the stressing
element (14) is a perforated disk which is arranged to rotate on the
reverse-gear gearwheel (4) and which has a through-hole (28) for each of
the blocking bolts (10).

23. The shifting assembly according to claim 22, wherein the perforated
disk (14) is pre-stressed, relative to the reverse-gear gearwheel (4), by
one of a spiral compression spring and a spiral tension spring.

24. The shifting assembly according to claim 14, wherein the reverse-gear
gearwheel (4) is forced in an axial direction, relative to the
transmission shaft (2), for engaging and disengaging the reverse gear.

25. The shifting assembly according to claim 14, wherein the clutch body
(6), the synchronizer ring (23), the stressing element (14) and the
blocking element (10) are arranged, at least in an engaged condition of
the reverse gear, within an outer contour of the reverse-gear gearwheel
(4) and a forward-gear gearwheel (15) adjacent to the reverse-gear
gearwheel.

26. A shifting assembly (1) in combination with a variable-speed
gearwheel transmission, the shifting assembly for shifting a reverse gear
of the variable-speed gearwheel transmission, the shifting assembly
comprising: a reverse-gear gearwheel (4) arranged to rotate on a
transmission shaft (2) and having a first clutch tooth array (5); a
clutch body (6) connected in a rotationally fixed manner to the
transmission shaft (2) and having a second clutch tooth array (7)
associated with the reverse-gear gearwheel (4); the first and the second
clutch tooth arrays (5, 7) are brought into engagement with one another
for engaging the reverse gear; a synchronizer ring (8) having a friction
surface being associated with the reverse-gear gearwheel (4), and the
synchronizer ring (8) co-operating with a counterpart friction surface
(16) during a synchronization process; the synchronizer ring (8) and the
reverse-gear gearwheel (4) being rotatable relative to one another, by a
limited angular amount in a circumferential direction, between a blocked
position and a through-connection position; at least one stressing
element (14) being arranged between the synchronizer ring (8) and the
reverse-gear gearwheel (4) and being actively connected to the
synchronizer ring (8) and the reverse-gear gearwheel (4) in such manner
that the synchronizer ring (8) is pre-stressed relative to the
reverse-gear gearwheel (4) in a direction of the blocked position; and in
the blocked position at least one blocking element (10) prevents the
first and the second clutch tooth arrays (5, 7) from being forced into
engagement with one another by positive interlock means.

27. A shifting assembly (1) for shifting the reverse gear of a
variable-speed gearwheel transmission, the shifting assembly comprising:
a clutch body (6) being connected in a rotationally fixed manner to a
transmission shaft (2) and comprising a second tooth array (7); a
reverse-gear gearwheel (4) comprising a first tooth array (5) and being
rotatably supported and axially slidable on a transmission shaft (2)
between a disengaged position and an engaged position, and the first
tooth array (5) engaging with the second tooth array (7) when the
reverse-gear gearwheel (4) is in the engaged position; a synchronizer
ring (8) being associated with the reverse-gear gearwheel (4) and having
a friction surface which co-operates with a counterpart friction surface
(16) during a synchronization process; the synchronizer ring (8) and the
reverse-gear gearwheel (4) being rotatable relative to one another, by a
limited angular amount in a circumferential direction, between a blocked
position and a through-connection position; at least one stressing
element (14) being axially arranged between the synchronizer ring (8) and
the reverse-gear gearwheel (4) and actively connected to the synchronizer
ring (8) and the reverse-gear gearwheel (4) such that the synchronizer
ring (8) is pre-stressed relative to the reverse-gear gearwheel (4) in a
direction of the blocked position; and in the blocked position, at least
one blocking element (10) prevents engagement of the first and the second
tooth arrays (5, 7) with one another by positive interlock means.

[0002] The present invention concerns a shifting assembly for shifting the
reverse gear of a variable-speed gearwheel, and a variable-speed
gearwheel transmission with such a shifting assembly.

BACKGROUND OF THE INVENTION

[0003] For cost reasons and due to the restricted structural space
available, until now manual-shift and semi-automatic variable-speed
gearwheel transmissions have often been built and used without mechanical
synchronization of the reverse gear. In such cases, however, under
certain conditions when the reverse gear is engaged there occur
unpleasant rattling noises and shift jerks. Besides, rattling when the
reverse gear is engaged also causes increased wear of the gearwheels.
Furthermore, when the transmission is hot the shifting time can be quite
long.

[0004] To avoid these drawbacks a number of solutions have already been
proposed. However, none of these solutions is optimal because they entail
too much fitting space, high manufacturing cost or long shifting times.
From DE 100 54 757 A1 a shifting assembly for the rapid and noise-free
shifting of the reverse gear in a variable-speed gearwheel transmission
is known, which takes up very little structural fitting space. In that
document a shifting assembly with a reverse-gear gearwheel is described,
with which are associated a synchronizer ring and a clutch body with a
friction cone. The clutch body is connected in a rotationally fixed
manner to a transmission shaft. The synchronizer ring is provided with an
all-round groove in which a ring spring is set. An inner tooth array of
the reverse-gear gearwheel is also provided with an all-round groove
which, when the reverse gear is not engaged, is positioned opposite the
groove in the synchronizer ring and in which the ring spring partially
engages. To engage the reverse gear the spring force of the ring spring
first has to be overcome, whereby a friction cone of the synchronizer
ring is pressed against the friction cone of the clutch body so that
synchronization between the rotational speeds of the gearwheel and the
transmission shaft takes place. The spring force of the ring spring must
be chosen appropriately large in order to ensure sufficient
synchronization in any operating conditions and to prevent over-rapid
shifting and rattling.

[0005] With this shifting assembly, to engage the reverse gear the spring
force of the ring spring to be overcome must always be the same,
regardless of operating conditions such as the temperature and thus
regardless of whether the synchronization process is completed earlier or
later. The result of this is that with a shifting assembly according to
DE 100 54 757 A1 the shifting comfort under certain operating conditions
is limited, either by a marked shifting resistance if the ring spring is
strong, or by rattling noises if it is weaker.

SUMMARY OF THE INVENTION

[0006] Accordingly the purpose of the present invention is to provide a
shifting assembly for trouble-free, comfortable shifting of the reverse
gear in a variable-speed gearwheel transmission in any operating
conditions, which takes up as little space as possible and ensures
rattle-free shifting. In addition a corresponding variable-speed
gearwheel transmission is indicated.

[0007] A shifting assembly for shifting the reverse gear of a
variable-speed gearwheel transmission is proposed, which comprises a
reverse-gear gearwheel mounted to rotate on a transmission shaft. The
reverse-gear gearwheel has a first clutch tooth array. Associated with
the reverse-gear gearwheel is a clutch body connected in a rotationally
fixed manner to the transmission shaft, which has a second clutch tooth
array. To engage the reverse gear, the first and second clutch tooth
arrays are brought into engagement with one another, for example by being
pushed into one another in the axial direction. Actively connected to the
reverse-gear gearwheel is a synchronizer ring which has a friction
surface that co-operates with a counterpart friction surface during the
synchronization process.

[0008] According to the invention, the synchronizer ring and the
reverse-gear gearwheel can be rotated relative to one another in the
circumferential direction by a limited angular amount. In this case the
synchronizer ring and the reverse-gear gearwheel can be rotated relative
to one another between a blocking position and a through-connection
position. In addition at least one stressing element is arranged between
the synchronizer ring and the reverse-gear gearwheel, which pre-stresses
the synchronizer ring relative to the reverse-gear gearwheel in the
direction of the blocking position, whereby pushing of the two clutch
tooth arrays into one another in the blocking position is prevented by
positive interlock by means of at least one blocking element. In this
way, when the reverse gear is not engaged the shifting assembly is always
in the blocked position and over-rapid or inadvertent shifting into the
reverse gear is effectively prevented.

[0009] Thus, the shifting assembly according to the invention has the
advantages of a conventional synchronization system, which needs fewer
components and takes up correspondingly less space. During the
synchronization process, i.e. so long as the rotational speeds to be
equalized are not yet equal, the stressing force acting on the stressing
element and the friction force produced at the friction surfaces due to
the speed difference add up to a locking force that holds the shifting
assembly in the blocked position and thereby prevents a premature,
damaging shift. Thus the acting locking forces are the force in the
circumferential direction produced by the pre-stressed stressing element
and the force in the circumferential direction produced by the
synchronization process between the friction surface and the counterpart
friction surface.

[0010] The synchronization process begins when, as the reverse gear is
being engaged, the friction surface and its counterpart friction surface
begin producing a torque, and is completed when at least approximately
equal rotational speeds have been reached. After the completion of the
synchronization process, i.e. when the speeds of the reverse-gear
gearwheel and the clutch body are at least approximately the same, the
blocked position can be released using a small shifting force since the
stressing element and thus the at least one blocking element can be
rotated relative to the reverse-gear gearwheel to a through-connection
position. During this the pre-stressing force of the stressing element is
overcome. In the through-connection position the first clutch tooth array
on the reverse-gear gearwheel is brought into engagement with the second
clutch tooth array on the clutch body, so that torque can be transmitted
by the reverse-gear gearwheel to the transmission shaft. The
pre-stressing force of the stressing element can be kept relatively
small.

[0011] The first clutch tooth array on the reverse-gear gearwheel can
either be made integrally with the gearwheel, or it can be formed on a
clutch body that is connected in a rotationally fixed manner to the
reverse-gear gearwheel.

[0012] In a first preferred design of the invention the counterpart
friction surface that co-operates with the friction surface of the
synchronizer ring is located on a forward-gear gearwheel adjacent to the
reverse-gear gearwheel. This design involves particularly few components
and enables the space taken up to be even smaller.

[0013] Since the reverse-gear gearwheel and the adjacent forward-gear
gearwheel are necessarily coupled by the countershaft in opposite
rotational directions, in this embodiment the common rotational speed can
only be zero. As a rule, in variable-speed gearwheel transmissions the
gearwheel for the first gear is arranged next to the reverse-gear
gearwheel, and in this design it additionally serves as the counterpart
friction surface.

[0014] According to a second preferred design, the counterpart friction
surface is formed on a synchronizer element that is connected in a
rotationally fixed manner to the transmission shaft. This synchronizer
element can be formed as a separate ring or integrally with the clutch
body, and has a conical counterpart friction surface.

[0015] A further preferred design provides that the at least one blocking
element is in the form of a plurality of blocking bolts distributed
uniformly around the circumference of the synchronizer ring. This
blocking bolt synchronizer design enables a further optimized use of
structural space, in particular also because when the reverse gear is
engaged the blocking bolts are pushed into associated bores in the
reverse-gear gearwheel.

[0016] The synchronizer ring can for example be connected to the
reverse-gear gearwheel with positive interlock so that the synchronizer
ring rotates together with the reverse-gear gearwheel around the
transmission shaft. However, the interlocked connection between the
synchronizer ring and the reverse-gear gearwheel allows at least as much
play in the circumferential direction as is necessary to enable the two
components to be rotated between the blocked position and the
through-connection position. Here, the rotational axis corresponds to the
central axis of the transmission shaft. According to a further preferred
feature of the invention, the at least one cylindrical blocking bolt has
at its free end a cylindrical drive-pin whose diameter is smaller than
that of the rest of the blocking bolt and which projects into a bore
associated with the drive-pin in the reverse-gear gearwheel.

[0017] Preferably, the transition between the larger diameter of the
blocking bolt and the smaller diameter of the drive-pin is in the form of
a conical blocking surface.

[0018] Another aspect of the invention concerns the stressing element that
pre-stresses the synchronizer ring toward the reverse-gear gearwheel and
holds it in the blocked position until the shift actuation force when the
reverse gear is engaged after the synchronization process overcomes the
stressing force and rotates the synchronizer ring with its blocking bolts
relative to the reverse-gear gearwheel, into the through-connection
position. The stressing element is preferably in the form of a perforated
disk arranged to rotate on the reverse-gear gearwheel and having a
through-hole for each of the blocking bolts. Preferably, the perforated
disk is pre-stressed relative to the reverse-gear gearwheel by means of a
spiral compression spring or a spiral tension spring.

[0019] To engage and disengage the reverse gear, the reverse-gear
gearwheel is preferably pushed in the axial direction relative to the
transmission shaft in order to bring the clutch tooth arrays respectively
into or out of mutual engagement with one another. Compared with
conventional synchronized shifting assemblies this saves a separate
sliding sleeve and synchronizing body, and this above all substantially
reduces the fitting space in the axial direction. It is also conceivable
that instead of the reverse-gear gearwheel, it is the clutch body which
is displaced axially in order to bring the clutch tooth arrays into
engagement with one another.

[0020] Thanks to the, space-saving design features it is made possible for
the clutch body, the synchronizer ring, the stressing element and the
locking element to be arranged within the outer contour of the
reverse-gear gearwheel and the forward-gear gearwheel adjacent to the
reverse-gear gearwheel. This means that the proposed shifting assembly
with synchronization of the reverse gear is actually of a form that is
neutral as regards structural fitting space, i.e. compared with shifting
assemblies with a non-synchronized reverse gear no additional fitting
space is required. In a conventional synchronization unit with a
synchronizer body and a sliding sleeve this is not possible, because the
components require additional space particularly in the axial direction.

[0021] The invention also includes shifting assemblies with
synchronization systems comprising double-cone or multiple-cone
synchronizers known per se.

[0022] Finally, a variable-speed gearwheel transmission is claimed, which
comprises a shifting assembly of the type described above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Example embodiments of the shifting assembly according to the
invention are illustrated in the attached figures and described in more
detail below. The figures show:

[0024] FIG. 1: Part of a first embodiment of the shifting assembly
according to the invention, with the reverse gear engaged, represented as
a schematic sectional view,

[0025] FIG. 2: Part of a second embodiment of the shifting assembly
according to the invention, with the reverse gear disengaged, represented
as a schematic sectional view,

[0026] FIG. 3: Part of the second embodiment of the shifting assembly
according to the invention, with the reverse gear engaged, represented as
a schematic sectional view,

[0027] FIG. 4: Part of a shifting assembly according to the invention with
a schematic sectional representation of the active connection between the
reverse-gear gearwheel, a blocking element and the stressing element,
with the reverse gear disengaged, and

[0028] FIG. 5: Part of a shifting assembly according to the invention with
a schematic sectional representation of the active connection between the
reverse-gear gearwheel, a blocking element and the tensioning element,
with the reverse gear engaged.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] In the embodiment of the shifting assembly 1 according to the
invention shown in FIG. 1, a reverse-gear gearwheel 4 is arranged so that
it can rotate and move axially on a transmission shaft 2. The
reverse-gear gearwheel rotates about the central axis 3 of the
transmission shaft 2 and is mounted on the transmission shaft 2 by means
of a roller bearing 17. In the area of the outer circumference of the
reverse-gear gearwheel 4 it has a groove 19 in which the end of a
shifting fork 20 engages. Next to the reverse-gear gearwheel 4 a
forward-gear gearwheel 15 is also mounted via a roller bearing 18 to
rotate about the central axis 3 of the transmission shaft 2.

[0030] On the side of the reverse-gear gearwheel 4 facing away from the
forward-gear gearwheel 15 is arranged on the transmission shaft 2 a
roller bearing 21 by means of which the transmission shaft 2 is mounted
in a transmission housing (not shown). The transmission shaft 2 can be
for example the drive input shaft of the transmission. The reverse-gear
gearwheel 4 and the roller bearing 21 are supported in the axial
direction relative to the transmission shaft 2 by a locking ring 22.

[0031] Between the reverse-gear gearwheel 4 and the forward-gear gearwheel
15, a clutch body 6 is attached in a rotationally fixed manner on the
transmission shaft 2. It has a second clutch tooth array 7 on its outer
circumference, which in the condition illustrated, is engaged with a
first clutch tooth array 5 formed on the reverse-gear gearwheel 4.

[0032] Between the reverse-gear gearwheel 4 and the forward-gear gearwheel
15 there is also arranged a synchronizer ring 8 with a plurality of
blocking bolts 10 distributed around the circumference of the
synchronizer ring 8. In this example embodiment the blocking bolts 10 are
made integrally with the synchronizer ring 8. Only one of the blocking
bolts 10, can be seen in FIG. 1. In the engaged condition of the reverse
gear, as illustrated, the blocking bolt 10 projects into a bore 13 in the
reverse-gear gearwheel 4 associated with the blocking bolt 10. A
drive-pin 11 is arranged at the free end of the blocking bolt 10. From
its larger diameter, the blocking bolt 10 merges via a conical locking
surface 12 down to the smaller diameter of the drive-pin 11.

[0033] In the area of its inside diameter, the synchronizer ring 8 has a
conical friction surface 9 which, during the synchronization process,
co-operates with a counterpart friction surface 16 opposite it. In this
embodiment the counterpart friction surface 16 is arranged directly on
the forward-gear gearwheel 15.

[0034] A stressing element 14 in the form of a perforated disk 14 is
arranged between the reverse-gear gearwheel 4 and the forward-gear
gearwheel 15. The perforated disk 14 has a through-hole 28, through which
the blocking bolt 10 fixed on the synchronizer ring 8 projects into the
bore 13 of the reverse-gear gearwheel 4.

[0035] FIG. 2 shows part of a second embodiment of the shifting assembly
according to the invention, with the reverse gear disengaged. Since the
second embodiment has a structure similar to that of the first embodiment
shown in FIG. 1, the same indexes have been used for the same components
and features in FIGS. 1 and 2.

[0036] The essential difference of the second embodiment in FIG. 2
compared to the first embodiment in FIG. 1 is that the counterpart
friction surface 16 is not on the forward-gear gearwheel 15, but on a
separate ring 23 arranged in a rotationally fixed manner on the
transmission shaft 2 between the clutch body 6 and the forward-gear
gearwheel 15. For this, the ring 23 has on its outer circumference a
conical surface, which serves as the counterpart friction surface 16. As
in the first embodiment described above, the friction surface 9 arranged
on the synchronizer ring 8 co-operates with the counterpart friction
surface 16.

[0037] FIG. 2 shows the shifting assembly in the disengaged condition of
the reverse gear. Thus, the reverse-gear gearwheel 4 is in its position
furthest removed from the adjacent forward-gear gearwheel 15 and is over
the roller bearing 21. The first clutch tooth array 5 on the reverse-gear
gearwheel 4 is not engaged with the second clutch tooth array 7 on the
clutch body 6. Thus, in this condition there is no torque-transmitting
connection between the reverse-gear gearwheel 4 and the transmission
shaft 2. In this disengaged condition of the reverse gear, only the
drive-pin 11 of the blocking bolt 10 projects into the bore 13 in the
reverse-gear gearwheel 4. Since the drive-pin 11 has a smaller diameter
than the bore 13, the synchronizer ring 8 made integrally with the
blocking bolt 10 and drive-pin 11 is rotated by the pre-stressed
stressing element 14 in the circumferential direction by a certain
angular amount relative to the reverse-gear gearwheel 4. In this
condition the shifting assembly 1 is in its blocked position.

[0038] FIG. 3 shows the same embodiment as FIG. 2, but with the reverse
gear engaged. In this case the reverse-gear gearwheel 4 has been pushed
by the shifting fork 20 all the way to the left, so that the inside
diameter of the reverse-gear gearwheel 4 is above the clutch body 6. The
first and second clutch tooth arrays 5 and 7 are engaged in one another
and there is a torque-transmitting connection between the reverse-gear
gearwheel 4 and the transmission shaft 2. The blocking bolt 10 projects
with its larger diameter as well into the bore 13 of the reverse-gear
gearwheel 4. In this condition the shifting assembly 1 is in its
through-connection position.

[0039] The association and positioning of the reverse-gear gearwheel 4,
the blocking element 10 and the stressing element 14 with the reverse
gear disengaged and engaged, are shown respectively in FIG. 4 and FIG. 5,
which represent respectively a section with the blocking bolt 10 and a
section with the interlocked connection between the tensioning element 14
and the reverse-gear gearwheel 4.

[0040] FIG. 4 shows the two sections when the reverse gear is in its
disengaged and blocked position. The reverse-gear gearwheel 4 is in its
position pushed all the way over to the right. Thereby, the blocking bolt
10 integrally connected to the synchronizer ring 8 is pulled far enough
out of the bore 13 in the reverse-gear gearwheel 4 such that only the
drive-pin 11 located on the free end of the blocking bolt 10 still
projects into the bore 13. Since the drive-pin 11 has a smaller diameter
than the bore 13, the synchronizer ring 8 with the blocking bolt 10 and
drive-pin 11 are rotated by the stressing element 14 pre-stressed in the
circumferential direction, relative to the reverse-gear gearwheel 4, far
enough for the drive-pin 11 to rest against the inside surface of the
bore 13. A through-going bore 28 in the stressing element 14, though
which the blocking bolt 10 extends, is arranged offset relative to the
bore 13 in the reverse-gear gearwheel 4 by a certain angular amount. In
this blocked position an axial displacement of the reverse-gear gearwheel
4 in the direction toward the synchronizer ring 8 is prevented by the
blocking surfaces 12 and 27 resting against one another.

[0041] The lower part of FIG. 4 shows a possible manner in which the
pre-stress of the stressing element 14 can be applied on the reverse-gear
gearwheel 4. For this, the reverse-gear gearwheel 4 has a recess 24 in
its side surface facing toward the synchronizer ring 8 and the stressing
element 14. On the stressing element 14 is formed a carrier-strip 26,
which projects into the recess 24 and co-operates with a spiral
compression spring 25 arranged in the recess 24. For this, the spiral
compression spring 25 rests at one end against a side surface of the
recess 24 and its other end presses against the carrier-strip 26 of the
stressing element 14. The size of the pre-stressing force can be
influenced as desired by using spiral compression springs 25 with various
spring characteristics. In the design of the recess 24 and the selection
of the spiral compression spring 25, care must be taken that the possible
rotation angle between the stressing element 14 and the reverse-gear
gearwheel 4 allows at least a rotation between the blocked position and
the through-connection position.

[0042] FIG. 5 shows the same sections as FIG. 4, but with the reverse gear
engaged in the through-connection position. The reverse-gear gearwheel 4
in this position is pushed all the way to the left. Consequently, the
blocking bolt 10 integrally connected to the synchronizer ring 8 is
pushed far enough into the bore 13 in the reverse-gear gearwheel 4 for
the blocking bolt 10 to rest against the inside surface of the bore 13 in
the area of its larger diameter. The through-going bore 28 in the
stressing element 14 is then positioned approximately aligned with the
bore 13 in the reverse-gear gearwheel 4.

[0043] The function of the shifting assembly according to the invention is
described below with reference to a shifting sequence.

[0044] Before the reverse gear is engaged, the shifting assembly and its
components are in the blocked position shown in FIG. 2. The clutch tooth
arrays 5 and 7 are not engaged with one another because the reverse-gear
gearwheel 4, which can move axially on the transmission shaft 2, is, in
this position, pushed to the right. Consequently the blocking bolts 10
are pulled far enough out of the associated bores 13 in the reverse-gear
gearwheel 4 such that only their respective drive-pins 11 still project
into the bores 13. Due to the pre-stress on the stressing element 14, the
synchronizer ring 8 with the blocking bolts 10 rotate far enough relative
to the reverse-gear gearwheel 4, for the drive-pins 11 to rest against
the inner circumference of the bores 13 and for the blocking surfaces 12
and 27 to be opposite one another.

[0045] To engage the reverse gear, the reverse-gear gearwheel 4 is pushed
by the shifting fork 20 axially to the left over the transmission shaft
2. During this the blocking surfaces 12 and 27 come together after a
first, short displacement movement. The shift actuation force in the
axial direction is transferred via the blocking surfaces to the
synchronizer ring 8 and the friction surface 9 is thus pressed against
the counterpart friction surface 16 that is connected in a rotationally
fixed manner to the transmission shaft 2. The friction force between the
friction surface 9 and the counterpart friction surface 16 now ensures
that any rotational speed differences existing between the transmission
shaft 2 and the reverse-gear gearwheel 4 are equalized. During the
synchronization process the friction force acts together with the
stressing force of the spring 25 as a blocking force in the
circumferential direction, in such manner that relative to the
reverse-gear gearwheel 4 the synchronizer ring 8 is held in the blocked
position. Only when the speeds of the transmission shaft 2 and the
reverse-gear gearwheel 4 are at least approximately the same, do the
friction force and thus the locking force in the circumferential
direction decrease perceptibly and it becomes possible to overcome the
locking force by the shift actuation force. For this, the reverse-gear
gearwheel 4 is pushed further to the left so that the conical blocking
surfaces 12 and 27 slide past over one another, whereby the stressing
element 14 is rotated against the stressing force of the spring 25.
During this, the reverse-gear gearwheel 4 is rotated relative to the
synchronizer ring 8 and the transmission shaft 2 until the blocking bolts
10 can be pushed into the bores 13, so that at the same time the clutch
tooth arrays 5 and 7 are brought into mutual engagement. Thereby, a
torque-transmitting connection is formed by the reverse-gear gearwheel 4
and the transmission shaft 2.

[0046] In the embodiment according to FIG. 1, during the synchronization
process the rotational speeds of the reverse-gear gearwheel 4 and the
forward-gear gearwheel 15 for the first gear, arranged adjacent to it,
are synchronized. Since the two gearwheels 4 and 15 are connected in a
rotationally fixed manner to one another by a countershaft (not shown)
but have different rotation directions, the common rotational speed of
these gearwheels can always only be zero.